1. Field of the Invention
This invention relates to photographic elements containing silver halide crystals or grains as the light sensitive particles and to methods of controlling the sensitometric response curve especially in the toe of the curve, i.e., the part of the curve corresponding to the most sensitive crystals or sensitive reponse of crystals in the photographic film.
2. Background of the Art
The response of a photographic film to radiation exposure (either to wavelengths of native sensitivity or to regions of the electromagnetic spectrum to which the grains have been spectrally sensitized) is measured by a sensitometric curve. A sensitometric curve is traditionally a graphic representation of the relationship between the energy used to expose the photographic element (usually expressed as "logE," the logarithm of the energy) versus the optical density (usually "D" generated in the photographic element after photographic development). The film is exposed to a graduated light intensity and the film's response (usually measured as optical density after development) as a function of the light gradation is measured. The part of the response curve where the film first begins to respond to an exposure is called the toe of the curve. While most of the density of an image is produced at higher exposures than that needed to expose the toe area, there are important features of the image which are determined by the toe area. In films used for graphic arts purposes, the edges of a half tone dot may be controlled by the sensitometry of the toe area of the curve. A large toe area produces a low density shadow around the dot known as a soft dot and this is undesirable for films used to subsequently expose a printing plate.
Medical radiographic films, especially those exposed by laser imaging devices, exhibit most of their information in the main part of the sensitometric curve; but a long toe area can cause the appearance of blurred alphanumeric characters on the image and a higher background density in the regions of lowest exposure.
We have found that the shape of the toe area of such films can be modified by the addition to the photographic emulsion of a class of organic compound.